Optical rotational position information detecting apparatus

ABSTRACT

An apparatus for optically detecting rotational position information of a rotating object, includes a light source, a detector which is located at a position to receive scattered light from the rotating object when the rotating object is irradiated with a light beam from the light source, and outputs a frequency signal based on the scattered light, a signal processing system for detecting rotational position information by performing signal processing for the frequency signal from the detector, and a rotation control system for controlling rotation of the rotating object. The rotation control system preliminarily rotates the rotating object in detecting the rotational position information.

This is a division of application Ser. No. 09/704,704, filed Nov. 3,2000 now U.S. Pat. No. 6,829,118.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical rotational positioninformation detecting apparatus mainly used for an apparatus foroutputting rotational position signals required as clock signals for ahard disk drive, a photosensitive drum rotational position signal outputapparatus for a copying machine, or the like.

2. Related Background Art

FIG. 1A is a plan view of a conventional data writing/reading apparatusused for a data processing apparatus such as a computer. A magnetic disk2 formed by, for example, coating a disk with a magnetic material isplaced on an HDD (Hard Disk Drive) 1 serving as a data writing/readingapparatus. The HDD 1 has a magnetic head arm 4 having a magnetic headslider 3 mounted on its distal end. The magnetic head slider 3 writes aninformation signal on the magnetic disk 2. A voice coil motor 5 ismounted on the rear end portion of the magnetic head arm 4.

FIG. 1B is a plan view of the magnetic disk 2. FIG. 1C is a view forexplaining a servo pattern. A plurality of sectors, each consisting of aservo pattern region and data region, are arranged outside a disk hub 2a of the magnetic disk 2.

In this arrangement, the magnetic disk 2 is set on the HDD 1, and therotational position of the magnetic head arm 4 is forcibly set to aposition corresponding to a desired track by a rotary positioner (notshown). The magnetic head arm 4 is then driven in the track direction towrite a servo pattern of an information signal with a resolution 1/2 adata track using the magnetic head slider 3 on the distal end of thearm.

When a data signal is to be written on the circular magnetic disk 2 byusing the magnetic head slider 3, a servo track signal as informationfor positioning the magnetic head slider 3 must be accurately written inadvance. For this purpose, a magnetic signal must be written at adesired position on the magnetic disk 2 by accurately detecting positioninformation of the rotational direction of the magnetic disk 2 as wellas position information of the magnetic head slider 3 in the trackdirection which is the radial direction of the magnetic disk 2.

FIG. 1D is a perspective view of the HDD 1 having a magnetic clock head7 that is used independently of a magnetic head 6 for writinginformation to accurately detect the rotational direction of themagnetic disk 2. According to this scheme, the magnetic clock head 7enters the HDD 1 through an opening portion 8 and writes a clock signalof a rotational direction on an outermost peripheral portion of themagnetic disk 2. The rotational position of the magnetic disk 2 is thendetected while the clock signal is read by the magnetic clock head 7,and a servo track signal is written on each track using the magnetichead 6 for writing information.

The service life of this magnetic clock head 7 is short because it isused to write clock signals on several ten thousand HDDs 1 in a shortperiod of time. That is, the magnetic clock head 7 itself is aconsumable item, and hence maintenance such as replacement is required,resulting in an increase in cost.

In addition, since the gap between the magnetic disk 2 and the magneticclock head 7 must be kept very small, these members may contact eachother for some cause. This structure is therefore structurallyundesirable in efficiently and economically mass-producing HDDs 1.

As a means for solving this problem, a laser Doppler scheme of detectingthe rotational position information of a rotating object by irradiatingit with a laser beam is disclosed in Japanese Patent ApplicationLaid-Open No. 7-29229. According to this laser Doppler scheme, since itis only required to irradiate the disk hub 2 a of the magnetic disk 2with a laser beam, no special part such as a scale needs to be bonded tothe magnetic disk 2. In addition, owing to noncontact detection, thedetecting unit does not wear.

FIG. 1E is a view showing the arrangement of a laser Dopplervelocimeter. This device measures the moving velocity of a moving objectby using the Doppler effect that when the moving object is irradiatedwith a laser beam, the frequency of light scattered by the moving objectshifts in proportion to the moving speed. In this device, a laser source11, collimator lens 12, beam splitter 13, and mirrors 14 a and 14 b arearranged. An object K to be measured, which moves in the directionindicated by the arrow at a velocity V, is placed in the reflectiondirection of the two mirrors 14 a and 14 b, and a condenser lens 15 andphotodetector 16 are arranged on the optical path of light reflected bythe object K.

In this arrangement, a laser beam emitted from the laser source 11 iscollimated into a parallel light beam L1 by the collimator lens 12 andstrikes the beam splitter 13 to be split into two light beams L2 and L3.These light beams are reflected by the mirrors 14 a and 14 b and strikethe object K, which is moving at the velocity V, at an incident angle θ.Scattered light from the object K is detected by the photodetector 16via the condenser lens 15.

The frequency of the scattered light beams originating from the twolight beams respectively undergo Doppler shifts +Δf and −Δf. Letting λbe the wavelength of a laser beam, Δf is given byΔf=(V sin θ)/λ  (1)

The scattered light beams having undergone the Doppler shifts +Δf and−Δf interfere with each other to cause brightness changes on thelight-receiving surface of the photodetector 16. A frequency F at thistime is given byF=2Δf=(2V sin θ)/λ  (2)

If the Doppler frequency F of the photodetector 16 is measured accordingto equation (2), the velocity V of the object K can be obtained.

When the object K is a rotating object, the velocity V of the object Kis given byV=2πrW/60  (3)where r is the irradiation radius and W (rpm) is the rotationalvelocity.

Equation (2) is finally rewritten intoF=(πrW sin θ)/(15λ)  (4)

If equation (4) is converted into a pulse count N for one revolution,equation (4) is rewritten intoN=(4πr sin θ)/λ  (5)

By detecting this pulse signal, rotational position information can bedetected.

(1) It is, however, known that the above conventional optical rotationalposition information detecting means for detecting rotational positioninformation by using a Doppler signal causes dropouts that are portionsin which signal components are statistically omitted. It is thereforedifficult to accurately identify a rotational position.

(2) In the above prior art, an NRRO (Non-Repeatable Run Out)corresponding to about 0.1 μm occurs when the magnetic disk 2 of the HDD1 rotates. To write a stable servo signal, it is very important to forma clock signal while minimizing the influence of this NRRO.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the above problem (1)and provide an optical rotational position information detectingapparatus which accurately detects rotational position information in acircumferential direction on a rotating object that continuouslyrotates.

It is another object of the present invention to solve the above problem(2) and provide an optical rotational position information detectingapparatus which can perform accurate position detection by minimizingthe influence of the run out of a rotating object.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of a conventional HDD;

FIG. 1B is a plan view of a magnetic disk;

FIG. 1C is a view for explaining a servo pattern on the magnetic disk;

FIG. 1D is a perspective view of the HDD using a magnetic clock head;

FIG. 1E is a view showing the arrangement of a laser Dopplervelocimeter;

FIG. 2A is a perspective view of a hard disk drive according to thefirst embodiment;

FIG. 2B is a graph showing a Doppler signal corresponding to arotational position;

FIGS. 3A and 3B are graphs showing processed signals in the firstembodiment;

FIG. 4 is a view showing the flow of storage of dropouts;

FIG. 5 is a flow chart showing signal processing;

FIGS. 6A and 6B are graphs showing processed signals in the secondembodiment;

FIG. 7 is a flow chart showing signal processing;

FIG. 8 is a flow chart showing signal processing according to the thirdembodiment;

FIG. 9 is a view showing the arrangement of the main part of the fourthembodiment;

FIG. 10 is a view for explaining the positional relationship associatedwith the detection direction of an LDV optical head;

FIG. 11 is a view for explaining a non-repeatable run out;

FIG. 12 is a graph showing write angle deviations at LDV angles andmagnetic head positions;

FIG. 13 is a graph showing write angle deviations at LDV angles; and

FIG. 14 is a graph showing magnetic head positions and write angledeviations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail with reference to theembodiments shown in FIGS. 2A to 14.

FIG. 2A is a perspective view showing an HDD applied to a servo trackwriter according to the first embodiment. A magnetic disk 22 is placedon an HDD 21. The HDD 21 has a magnetic head 23 a which is mounted onthe distal end of a magnetic head arm 23 and has a slider serving as amagnetic recording head for writing an information signal on themagnetic disk 22. An LDV (Laser Doppler Velocimeter) optical head 24 fordetecting the rotational velocity of the magnetic disk 22 by irradiatinga disk hub 22 a of the magnetic disk 22 with a laser beam is placedabove the HDD 21.

The output of the LDV optical head 24 is sequentially connected to asignal processing logic circuit 25, rotational position detectingcircuit 26, sector servo pattern writing circuit 27, and magnetic head23 a. The output of a track direction position control circuit 28 isconnected to the sector servo pattern writing circuit 27 and a rotarypositioner (not shown).

With this arrangement, the track direction of the magnetic head 23 a issequentially controlled by the rotary positioner (not shown) inaccordance with each track to write a servo track signal for each trackon the magnetic disk 22 on the basis of the rotational positioninformation of the magnetic disk 22 which is obtained from the signalprocessing logic circuit 25 for processing a Doppler signal from the LDVoptical head 24.

FIG. 2B(a) shows the amplitude of a Doppler signal from the LDV opticalhead 24 which corresponds to the rotational position. This signalamplitude exhibits repeatability with respect to the rotational positionas long as the laser beam irradiation region of the disk hub 22 aremains unchanged. FIG. 2B(b) shows a Doppler signal A, awaveform-shaped signal B obtained by converting the Doppler signal A,and a rotational position origin signal (rotational position index) C.The frequency of the Doppler signal A is proportional to the rotationalvelocity as long as the radius of the disk hub 22 a at the laser beamirradiation position always remains the same.

FIGS. 3A and 3B are graphs showing a signal processing method for adropout portion of a Doppler signal in the first embodiment. Referringto FIG. 3A, a signal (1) indicates the envelope (repeatability) of aDoppler signal, and a signal (2) is a comparate signal. This comparatesignal (2) is obtained by removing noise components from the Dopplersignal (1) upon providing a threshold with hysteresis. A dropout can bedetected by using the phenomenon that the comparate signal (2) cannotfollow the Doppler signal (1) at a dropout portion. However, the signalamplitude is small at the position of a dropout, and the phase andamplitude of the Doppler signal (1) change even with slight vibrations.For this reason, even if this signal is waveform-shaped, itrepeatability cannot be ensured.

FIG. 3B is a graph showing signal processing for identifying a dropoutportion. First of all, a 1/4 frequency signal (5) of a PLL oscillationsignal (7) is synchronized (phase-locked) to a Doppler comparate signalin the interval between a rotational position origin signal which is nota dropout portion and the first dropout portion. Subsequently, a dropoutidentification signal (3) including a dropout unstable portion like theone shown in FIG. 3A is stored by counting the pulses of the PLLoscillation signal (7) from the rotational position origin signal(rotational position index).

At a dropout portion, the voltage value of a VCO (not shown) used forthe PLL oscillation signal (7) is fixed and oscillated at an oscillationfrequency immediately preceding the dropout. Since the dropout portioncorresponds to a distance of several 10 μm, the rotational velocitiesbefore and after the dropout can be regarded as almost equal. Bycounting the pulses of the PLL oscillation signal (7) during thedropout, therefore, the dropout identification signal (3) exhibitinggood repeatability of the dropout start and avoidance timings can begenerated.

As is obvious from a signal (4) and the signal (5) in FIG. 3B, the phaseof a Doppler signal before a dropout often differs from that after thedropout. To prevent such a phase shift in signal processing, the PLLoscillation signal (7) obtained by multiplying the frequency of aDoppler comparate signal by two or more (four) is used as a signal fordetecting dropout avoidance.

Assume that the PLL reference signal obtained by this method is thesignal (4) obtained by ORing the signals (2) and (3). A phase comparisonsignal (6) is almost synchronized to the rising point of the signal (4)after the dropout avoidance, and the number of pulses of the PLLoscillation signal (7) switched from the dropout identification signal(3) is stored. At a portion other than a dropout, the phase comparisonsignal (6) synchronizes (phase-locks) the 1/4 frequency signal (5) ofthe PLL oscillation signal (7) to the Doppler comparate signal (4).

There are several 10 dropout portions during one revolution. For thisreason, the numbers of PLL oscillation signal pulses in regions whichare not dropouts and dropout regions are sequentially stored, withreference to the rotational position origin, so as to cover no dropoutuncertain regions, and dropout portions corresponding to one revolutionare identified, thereby determining the final dropout identificationsignal (3).

FIG. 4 shows the flow of operation of sequentially storing dropouts.First of all, a count value from the index of the PLL oscillation signal(7) to a first dropout D.O.1 is stored at the first revolution. Withthis operation, a PLL oscillation signal free from the instability ofD.O.1 can be obtained. A count value from the index of the PLLoscillation signal (7) corresponding to a next dropout D.O.2 is storedafter the above processing is performed at the second revolution byusing the dropout identification signal (2), and the PLL oscillationsignal in the interval between the index and dropout D.O.2 isstabilized. This makes it possible to obtain the PLL oscillation signal(7) free from the instability of the dropouts D.O.1 and D.O.2.

Likewise, n D.O. portions that exist during one revolution aresequentially stored at D.O.3, D.O.4, . . . , D.O.n, i.e., the thirdrevolution, fourth revolution, . . . , nth revolution, therebydetermining a last dropout identification signal corresponding to onerevolution.

As described above, if a dropout portion can be fixed, instability,i.e., random signal phase changes before and after a dropout, can beeliminated. If a dropout identification signal (9) from the rotationalposition origin can be determined, the PLL reference signal (4) that isstable throughout one revolution can be generated by ORing the Dopplercomparate signals (2) and (3) detected in real time. In addition, thePLL phase comparison signal (6) that is stable throughout one revolutionis generated from the PLL oscillation signal (7) and signal (3).

By performing PLL oscillation using the PLL reference signal (4) andphase comparison signal (6), therefore, the PLL oscillation signal (7)that is stable throughout one revolution can be generated, thusobtaining a stable signal equivalent to an encoder signal.

PLL oscillation pulses for one revolution can be determined by countingthem in response to several rotational position origins as triggers.Subsequently, a rotational position origin can be detected/generated bycounting the number of PLL oscillation pulses. This will obviate thenecessity for an external origin signal. In addition, in this case, thesame signal processing is always performed at the same rotationalposition, and closed processing (closed loop) is performed for onerevolution, thus making signal processing for PLL oscillation morestable and realizing good signal repeatability.

FIG. 5 is a flow chart showing a signal processing logic R3, in which anorigin signal is written as an external original signal on the magneticdisk 22 by the magnetic head 23 a before a servo track signal iswritten, and the origin signal is read by the magnetic head 23 a. Inthis case, after a dropout identification signal for one revolution ispreliminarily determined by rotating the magnetic disk 22, an originsignal can be generated by the signal processing logic R3 of the LDVoptical head 24. Therefore, no external origin signal is required towrite a servo track signal. In setting an external original signal,stable switching to the origin signal can be performed by the signalprocessing logic R3 by selecting a portion which is not a dropoutregion. According to this method, since no external origin sensor isrequired, a servo track write with a simple arrangement can beimplemented, thus providing a practical system.

FIGS. 6A and 6B are graphs showing a signal processing method accordingto the second embodiment. This signal processing method is used toaccurately detect a rotational position even if a Doppler signalundergoes a dropout state.

Referring to FIG. 6A, a signal (1) is a Doppler signal, and a signal (2)is a comparate signal. This comparate signal (2) is set at HI at a zerolevel, i.e., a falling point, of the signal (1) and set at LO at athreshold with hysteresis, i.e., a rising point, of the signal (1), thusremoving noise components from the Doppler signal (1). A signal (3) isobtained by advancing the Doppler signal (1) by 3/16 phase and setting athreshold to be smaller than that of the comparate signal (2).

FIG. 6B is a graph showing signal processing performed upon detection ofa dropout portion. In detecting a dropout, when the signal (2) is keptat H level, a dropout start is detected, and when the signal (2) goes toL level, a dropout end is detected. In addition, when the signal (2) isset at H level at a rising point of the signal (3), a dropout start isdetected. When the signal (2) is set at L level, a dropout end isdetected. This makes it possible to always detect a dropout at thetiming preceding a rising point of the signal (2) by 3/16 phase. Notethat a signal (4) is a dropout detection signal.

First of all, a 1/8 frequency signal (5) of a PLL oscillation signal (7)is synchronized (phase-locked) to a Doppler comparate signal in theinterval between a rotational position original signal corresponding toa non-dropout portion and the first dropout portion. At a dropoutportion, i.e., when the dropout detection signal (4) is at H level, thevoltage value of a VCO (Voltage-Controlled Oscillator) (not shown) usedfor the PLL oscillation signal (7) is fixed and oscillated at anoscillation frequency immediately preceding the dropout. Morespecifically, a signal (6) generated by resetting the 1/8 frequencysignal of the PLL oscillation signal (7) as a PLL phase comparisonsignal using the dropout detection signal (4) is used to prevent thesignal (2) serving as a PLL reference signal and the phase comparisonsignal (6) from rising at the time of the dropout. This makes the VCOkeep oscillating without frequency adjustment of the PLL oscillationsignal (7).

Since the dropout portion corresponds to a distance of several 10 μm,the rotational velocities before and after the dropout can be regardedas almost equal. In this case, the comparison between the signals (2)and (5) reveals that the phase of the Doppler signal (1) before thedropout differs from that after the dropout. In this embodiment, topreliminarily detect this phase shift by signal processing, detection isperformed after dropout avoidance at the timing preceding a rising pointof the comparate signal (2) by 3/16 phase. The signal (6) to besynchronized to the comparate signal (2) is generated at the third pulseof the 8-times frequency-multiplied PLL oscillation signal (7) after afalling point of the detection signal (4). The PLL oscillation signal(7) is then frequency-divided by 8 until the next dropout, therebyachieving phase locking.

The PLL phase comparison signal (6) obtained by this method may shiftfrom the PLL reference signal (2) by a quantization error (±1/16 phase)at a rising point after dropout avoidance. However, as the PLLmultiplication number is increased, the error can be neglected.

When the PLL phase comparison signal (6) is generated withoutpreliminary detection, the quantization error becomes a positive 1/8phase, and the quantization error accumulation always increases. Incontrast to this, if dropouts are preliminarily detected as in thisembodiment, quantization errors can be assigned to positive and negativevalues. This makes it possible to bring the average of quantizationerror accumulations to zero without increasing them in one direction.

In general, the number of dropouts for one revolution is 100 or less.If, therefore, the PLL multiplication number is 128, a variation in thenumber of PLL oscillation pulses for one revolution (rotation detectionerror) is not ±1/2 pulse or more at maximum in terms of the number ofpulses of the Doppler signal (1). This makes it possible to set a gateat the position of a rising point of the Doppler signal (1) at the samerotational position with the number of PLL oscillation pulses andgenerate a rotational position original signal on the basis of a risingpoint of the Doppler signal (1) having passed through the gate. If thenumber of pulses of the PLL oscillation signal (7) is reset to coincidewith this rotational position origin signal, a rotational position canbe detected by reading the number of PLL oscillation pulses, thusimplementing a noncontact-type rotational position detecting systemrequiring no scale.

FIG. 7 is a flow chart showing a signal processing logic R1, in which anorigin signal is written as an external original signal on the magneticdisk 22 by the magnetic head 23 a before a servo track signal iswritten, and the original signal is read by the magnetic head 23 a. Inthis case, the magnetic disk 22 is preliminarily rotated to check thenumber of PLL oscillation signal pulses for one revolution, and a gateis set at the position of a rising point of the Doppler signal (1) atthe same rotational position. With this operation, since an originsignal can be generated by the signal processing logic R1 using the LDVoptical head 24, no external origin signal is required to write a servotrack signal. By reading the number of PLL oscillation pulses in thismanner, a rotational position is detected.

FIG. 8 is a flow chart showing a signal processing logic R2 according tothe third embodiment, in which an origin signal is written as anexternal original signal on a magnetic disk 22 by a magnetic head 23 abefore a servo track signal is written, and the original signal is readby the magnetic head 23 a. In this case, the magnetic disk 22 ispreliminarily rotated to check the number of PLL oscillation pulses forone revolution, and at the same time, a dropout portion corresponding toa rotational position is checked and roughly stored with the number ofPLL oscillation pulses.

Subsequently, an index signal for each of a plurality of sectors (about50 in general) is set at a rising point of a Doppler signal (1)independently of an external origin signal. If the index positioncoincides with the stored dropout portion, an index signal is set at aportion which is not the most recent prior dropout. When the number ofPLL oscillation pulses is reset in accordance with this index signal, areset PLL oscillation signal (7) does not include any instability ofsignal processing due to a dropout but includes only a quantizationerror (±1/16 phase) of a PLL phase comparison signal (6). By readingboth the count number of pulses of this index rotational signal and thecount number of PLL oscillation pulses, a rotational position can beaccurately detected.

In the second embodiment, a quantization error accumulation for onerevolution can become a rotational position detection error. In contrastto this, this embodiment is configured to disperse a quantization erroraccumulation to a fraction of the number of sectors (about 50), andhence is effective especially when a strict rotational positiondetection accuracy is required. By using this method, even in theDoppler signal (1) including dropouts, rotational position detectionwith very high repeatability can be realized without excessivelyincreasing the frequency of the PLL oscillation signal (7). In thisembodiment, when a sector index signal is identified, no external originsignal is required.

FIG. 9 is a perspective view showing the main part of the fourthembodiment, in which the present invention is applied to a system fordetecting the outer surface moving distance of a photosensitive drum 31.The output of an LDV optical head is connected to the photosensitivedrum 31 via a signal processing logic circuit 33, outer surfacerotational position detecting circuit 34, and rotational driving controlcircuit 35.

Conventionally, the outer surface moving amount of the photosensitivedrum 31 is detected by using a rotary encoder. In this method, however,if the outer surface of the photosensitive drum 31 is eccentric withrespect to the rotational axis, the detected moving amount from therotary encoder deviates from the actual outer surface moving amount ofthe photosensitive drum 31 in proportion to a radial error. Therefore, amechanical arrangement with no eccentricity is required. For thisreason, in this embodiment, a signal from the LDV optical head isdetected by the outer surface rotational position detecting circuit 34via the signal processing logic circuit 33 to make the rotationaldriving control circuit 35 control the rotation of the photosensitivedrum 31. As described above, since an outer surface moving amount isdetected by using a light beam from the LDV optical head, rotationalposition information from which eccentricity is removed can be quicklydetected.

This embodiment presents a method of controlling the rotational drivingof the photosensitive drum 31. For example, the accuracy of finaltransfer characteristics can be improved by performing feedback todriving control on another transfer system or feedback to control on anexposure process. This processing can be performed in the same manner asa signal processing logic R3 in FIG. 12. In addition, a signal from alow-resolution rotary encoder (not shown) is used as an external signalto accurately detect an outer surface rotational moving amount.

The fifth embodiment will be described next. An apparatus of thisembodiment has the same arrangement as that of the first embodiment inFIG. 2A, and hence an illustration of this arrangement will be omitted.An RRO (Repeatable Run Out) and NRRO (Non-Repeatable Run Out) arepresent on a magnetic disk 22, and the RRO occurs in the same manner atthe same rotational position. Even if, therefore, this RRO is present asa detection error, since identical detection errors are superimposed onthe respective tracks, no write error occurs when a servo track signalis written. On the other hand, detection errors due to the NRRO arerandomly superimposed on the respective tracks. To accurately write aservo track signal, therefore, it is important to minimize the detectionerrors due to the NRRO.

FIG. 10 is a plan view showing the positional relationship between themagnetic disk 22, a magnetic head 23 a, and the detection direction ofan LDV optical head 24. Assume that the direction of the magnetic head23 a with respect to the center of the magnetic disk 22 is the x-axis.In this case, letting Ex be a vibration component in the x-axisdirection with respect to a vibration width E of the NRRO, Ey be avibration component in the y-axis direction, α be the angle of aperpendicular line in an LDV detection direction F with respect to thex-axis, and r be the radius of the magnetic disk 22 at an LDV detectionposition D, a detection error component S of the NRRO which exerts aninfluence in the LDV detection direction F is given byS=−Ex·sin α+Ey·cos α  (6)

A radial error component V of a run out NR which exerts an influence inthe perpendicular direction of LDV detection is given byV=Ex·cos α+Ey·sin α  (7)

Therefore, letting R be the distance between the rotational center ofthe magnetic disk 22 and the magnetic head 23 a, an LDV detection errorE due to the NRRO on the magnetic head 23 a is given byE=(R/r)·S·(r+V)/r  (8)

In this case, since V≦E<<r, equation (8) can be approximated as follows:E∝(R/r)·S=(R/r)·(−Ex·sin α+Ey·cos α)  (9)

If this write position deviation is converted into a write angledeviation ω, thenω=W/R=(1/r)·(−Ex·sin α+Ey·cos α)−Ey/R  (10)

In general, the NRRO has a direction-independent vibration width E, andits angle β takes a random value satisfying 0≦β<2π. This indicates thatthe center of the magnetic disk 22 randomly deviates within the rangeindicated by the hatched portion in FIG. 11.

In this case, the vibration component Ex of the run out NR in the x-axisdirection and the vibration component Ey in the y-axis direction areexpressed with E and β as follows:Ex=E·cos β, Ey=E·sin β  (11)

A substitution of equation (11) into equation (10) yieldsω=(E/r)·{−cos β·sin α+sin β·(cos α−r/R)}  (12)

In this case, since the angle β can take a random value within the rangeof 0≦β<2π, the write angle deviation |ω| due to the NRRO at the mountingangle α of the LDV optical head 24 can take a value that is equal to themaximum value at an angle β in equation (13):|ω|=|(E/r)·{−cos β·sin α+sin β·(cos α−r/R)}|  (13)

A condition required for the angle β in equation (13) to take a maximumvalue is dA/dβ=0 according to equation (14) below:A=−cos β·sin α+sin β·(cos α−r/R)  (14)

Therefore, dA/dβ can be written intodA/dβ=sin β·sin α+cos β·(cos α−r/R)=cos(β−α)−(r/R)·cosβ=sin(β+π/2−α)−(r/R)·cos β

According to equations (15), equation (16) is established:c=sin(π/2−α)−r/R=cos α−r/Rd=cos(π/2−α)=sin αsin Φ=c/(c ² +d ²)^(1/2)cos Φ=d/(c ² +d ²)^(1/2)  (15)dA/dβ=(c ² +d ²)^(1/2)·sin(β+Φ)  (16)

If dA/dβ=0, then β=−Φ according to equation (16).

According to equations (15), a condition required for the angle β inequation (13) to take a maximum value is expressed bysin β=−c/(c ² +d ²)^(1/2)cos β=d/(c ² +d ²)^(1/2)  (17)

A substitution of equation (17) into equation (14) yieldsA={−d·sin α−c·(cos α−r/R)}/(c ² +d ²)^(1/2)=−{sin²α+(cos α−r/R)²}/{sin ²α+(cos α−r/R)² }^(1/2)={sin²α+(cos α−r/R)²}^(1/2)  (18)

With operation, a maximum value ωmax of a write angle deviation |ω| byNRRO is given byωmax=|−(E/r)·{sin² α+(cos α−r/R)²}^(1/2)|=(E/r)·{1+(r/R)²−2·(r/R)·cosα}^(1/2)  (19)

As is obvious from equation (19), if R is a constant, ωmax is minimumwhen α=0 rad.

Consider specific numerical values for the 6.3-cm (2.5-inch) HDD 21.Assuming that r=10 mm, E=0.1 μm, and 14 mm≦R≦30 mm (if a=R/r, then1.4≦a≦3), the write angle deviation ωmax is obtained as shown in FIGS.12 to 14. The range of R indicates the movement of the magnetic head 23a in the disk track radial direction.

FIG. 12 shows three-dimensionally the write angle deviation ωmax at aposition a of the magnetic head 23 a. FIG. 13 shows the relationshipbetween the LDV angle α and the write angle deviation ωmax at a=1.4,2.2, and 3. FIG. 14 shows the relationship between the position a of themagnetic head 23 a and the write angle deviation ωmax with α=0, π/6,π/3, π/2, 2π/3, 5π/6, and π rad.

According to these results, even when the position a of the magnetichead 23 a moves within the range of 1.4 to 3, the write angle deviationωmax becomes minimum when α=0 rad, and becomes maximum when α=π rad. Thevalue of the write angle deviation ωmax does not depend on the sign of αas long as the absolute value of the LDV angle α remains the same.

As is obvious from the above description, when the detection directionof the LDV optical head 24 is near a direction perpendicular to astraight line connecting the center of the rotating object and themagnetic head 23 a, and the detection position is on the magnetic head23 a side with respect to the center of the rotating object, a clocksignal can be formed by the LDV optical head 24 which is least affectedby a detection error due to the run out of the rotating object.

As described above, by rotating the rotating object before therotational position information detecting means identifies rotationalposition information, information for identifying rotational positioninformation can be detected. This makes it possible to accurately detecta stable rotational position even with a frequency signal that causesdropouts.

In addition, by setting the detection direction of the rotationdetection position information detecting means to be near a directionperpendicular to a straight line connecting the center of the rotatingobject and the information recording head, and also setting thedetection position on the information recording head side with respectto the center of the rotating object, accurate position detection can beperformed with minimum influence of the run out of the rotating object,and a stable servo track signal can be written in the laser Dopplerscheme.

1. An apparatus for optically detecting rotational position informationof a rotating object on which information is recorded by an informationrecording head, comprising: a light source; a detector which is locatedat a position to receive scattered light from the rotating object whenthe rotating object is irradiated with a light beam from said lightsource, and outputs a frequency signal based on the scattered light; anda signal processing system for detecting rotational position informationby performing signal processing for the frequency signal from saiddetector, wherein a detection direction of the detector for therotational position information is near a direction perpendicular to astraight line connecting a rotational center of the rotating object andthe information recording head, and a detection position for therotational position information is on the information recording headside with respect to the center of the rotating object.
 2. Aninformation recording apparatus for recording information on a rotatingobject by using an information recording head, comprising: informationrecording means for recording information on the rotating object throughthe information recording head; and rotational position informationdetecting means for detecting rotational position information of therotating object, said rotational position information detecting meansincluding: (1) a light source; (2) a detector which is located at aposition to receive scattered light from the rotating object when therotating object is irradiated with a light beam from said light source,and outputs a frequency signal based on the scattered light; and (3) asignal processing system for detecting rotational position informationby performing signal processing for the frequency signal from saiddetector, wherein a detection direction of the detector for therotational position information is near a direction perpendicular to astraight line connecting a rotational center of the rotating object andthe information recording head, and a detection position for therotational position information is on the information recording headside with respect to the center of the rotating object.
 3. Aninformation recording method of recording information on a rotatingobject by using an information recording head, comprising: executinginformation recording for the rotating object through the informationrecording head; and performing rotational position information detectionfor the rotating object by using a rotational position informationdetecting apparatus, the rotational position information detectingapparatus including: (1) a light source; (2) a detector which is locatedat a position to receive scattered light from the rotating object whenthe rotating object is irradiated with a light beam from said lightsource, and outputs a frequency signal based on the scattered light; and(3) a signal processing system for detecting rotational positioninformation by performing signal processing for the frequency signalfrom said detector, wherein a detection direction of the detector forthe rotational position information is near a direction perpendicular toa straight line connecting a rotational center of the rotating objectand the information recording head, and a detection position for therotational position information is on the information recording headside with respect to the center of the rotating object.
 4. A method ofmanufacturing an information recording medium by recording informationon a rotating object using an information recording head, comprising:preparing a medium; executing information recording for the mediumthrough the information recording head; and performing rotationalposition information detection for the medium by using a rotationalposition information detecting apparatus, the rotational positioninformation detecting apparatus including: (1) a light source; (2) adetector which is located at a position to receive scattered light fromthe rotating object when the rotating object is irradiated with a lightbeam from said light source, and outputs a frequency signal based on thescattered light; and (3) a signal processing system for detectingrotational position information by performing signal processing for thefrequency signal from said detector, wherein a detection direction ofthe detector for the rotational position information is near a directionperpendicular to a straight line connecting a rotational center of therotating object and the information recording head, and a detectionposition for the rotational position information is on the informationrecording head side with respect to the center of the rotating object.